This study attempted to investigate the impact of teacher’s code-switching on the acquisition of a problematic grammatical structure, namely, object relative clauses, by intermediate EFL learners. Moreover, a secondary objective of the study was to determine the EFL learners’ attitudes and opinions regarding the effectiveness of teacher’s code-switching in their learning of a specific aspect of English grammar, object relative clauses. To meet this end, 54 EFL learners from the JameaLangsuage Institute in Tabriz were selected and took part in grammar and attitudes pre-tests and underwent the necessary instructions. Whereas in the experimental group, explicit grammar instruction was used along with the teacher’s code-switching, in the control group, explicit grammar instruction was given only in English which was the medium of instruction.Finally questionnaire and post-tests were administered. Two ANCOVAs were run on the collected data not only to compare the performance of both experimental and control groups after the treatment period, but also to show whether post-test differences were due to the effect of treatment – code-switching – or their differences in pre-tests. The results of the ANCOVAs revealed that teacher’s code-switching was positively effective both on learning problematic grammatical structure (i.e., object relative clauses) and learners’ attitudes toward the teacher’s code-switching. The results of the present study could help language teachers, teacher trainers, and policy makers to provide a better context for foreign language learning and improving different language skills.